The invention relates to an arrangement for regulating the combustion air proportions also known as air to fuel ratio and designated by the greek letter lambda .lambda., in the case of a gas engine intended for lean mixture operation, in the induction line of which there is an air-gas mixer and possibly a mixture compressor, a lambda controller downstream of which, for varying the combustion air proportions, there is a positioning device for adjusting the air-gas mixer, and also an output measuring device for ascertaining the engine output.
Where gas engines are concerned, it is known to use exhaust gas catalysts in order to reduce the emission of harmful substances, particularly the components NO.sub.x, CO and C.sub.n H.sub.m. When using a three-way catalyst to reduce the aforesaid harmful substance components NO.sub.x, CO and C.sub.n H.sub.m, the gas engine is operated within the range of a combustion air ratio of .lambda.=1, i.e. in the region of the stoichiometric combustion air ratio. In order to achieve a satisfactory conversion rate of the three-way catalyst for all three aforesaid harmful substance components and in order thus to achieve a reduction in the overall emission of pollutants, the combustion air proportion .lambda. must be regulated and maintained within a very narrow range, the so-called lambda window. For example, this lambda window may lie in the range from 0.986 to 0.990. Only when the lambda value is within this narrow range is optimum operation of the three-way catalyst assured.
In the case of a known arrangement, regulation is effected via a zirconium dioxide probe which measures in the waste gas the 0.sub.2 partial pressure in relation to the 0.sub.2 partial pressure of the environmental air. Despite a temperature compensation of the regulator, the narrow prescribed lambda window can scarcely be maintained in practice. Above all in the event of fluctuations in the gas composition such as occur with dump gas, and the fluctuations in heating output which are connected therewith, it is impossible to achieve an optimum efficiency in the known waste gas catalyst which is regulated via a lambda probe.
Further disadvantages of any arrangement working with a threeway catalyst reside in that any pollutants occurring in the gas can rapidly damage the catalyst and the probe and in that the consumption of the gas engines is relatively high. Furthermore, there is a high thermal loading on the engine components.
These last-mentioned disadvantages can be prevented by operating the gas engine on a lean mixture, with which it is possible to avoid the use of a waste gas catalyst. Operating on a lean mixture generally means that the gas engine is operated on a mixture having an increased combustion air ratio (e.g. .lambda.=1.3 to 1.7), in other words in which there is an excess of air over the stoichiometric quantity of air. When operating under such conditions, mainly the NO.sub.x pollutant components in the waste gas decrease rapidly with increasing .lambda.. But also the CO and HC waste gas components are intensely reduced in comparison with .lambda.=1 operation. In order to keep the NO.sub.x pollutant components in the waste gas below a required level (for example 500 mg/Nm.sub.3), therefore, it is necessary to operate with high combustion air ratios (e.g. .lambda.=1.6). If the lambda values are too high (if the mixture is too lean), combustion failures can occur. Therefore, the value of the combustion air proportions must even with lean operation, be kept within a lambda range, the bottom limit of which is determined by the required NO.sub.x pollutant emission and of which the upper limit represents the stalling limit.
The advantage over the narrow lambda window in the case of three-way catalysts, which lies within the region of the stoichiometric lambda value, resides in that the above-defined lambda range is with lean operation substantially wider (by factor of 20 or more) than the required lambda window for a three-way catalyst.
Typical widths of lambda range lie within the range from .DELTA..lambda..apprxeq.0.1, both the upper stalling limit and also the lower NO.sub.x pollutant emission limit falling with diminishing gas engine output. The location of the required lambda range and thus the desired value of the combustion air ratio which is established substantially in the middle of the lambda range will therefore vary as a function of the engine output. The width of the lambda range certainly reamins substantially constant over the engine output.
By reason of varying marginal conditions, the combustion air proportions .lambda. can for all identically adjusted air gas mixtures be displaced so that even under conditions of lean operation, a lambda control is needed which must however be more easily conducted by reason of the aforesaid wider lambda range. We can regard as "external" factors which influence the combustion air ratio .lambda. (for a fixed air gas mixer setting): Composition of the (propellant) gas (.DELTA..lambda.max .apprxeq.7.2% ), temperature difference between gas and air (.DELTA..lambda.max .apprxeq.2% ), relative humidity in the induction air (.DELTA..lambda.max .apprxeq.1% ) and the accuracy of the gas-air mixer (.DELTA..lambda.max .apprxeq.1.3% ). In an extreme case, as the total of the estimates of maximum variation in one direction, as indicated in brackets, there may be a lambda shift of 11.5%, so that without any adjustment and instead of a present lambda value, it is possible to arrive at a lambda value at which the entire engine stalls or at which the lambda value drifts into the other direction so that the NO.sub.x pollutant emission increases, for example four times the admissible NO.sub.x pollutant emission level.
In the case of a gas engine which can be operated on a lean mixture, regulation via an oxygen probe which measures the 0.sub.2 content in the waste gas has already been suggested. However, in practice, such a manner of regulation, using an oxygen probe, presents many disadvantages. The characteristic curve of the oxygen probe is very flat in the lean-operation range, so that relatively large lambda variations result in only negligible variations in the probe output signals so that the accuracy of the control arrangement is negligible. In addition to the high costs of such an oxygen probe and the associated evaluating circuit, a further disadvantage resides in the fact that each oxygen probe has to be independently adjusted. Furthermore, it has been found that during prolonged operation of the gas engine, oxygen probes are susceptible to breakdown and a displacement of the probe characteristic can occur.
The object on which the invention is based resides in providing a favourably costed arrangement of the type mentioned at the outset, for regulating the combustion air mixture .lambda. for a gas engine operating under lean conditions, by which, without using susceptible and expensive oxygen probes in the waste gas pipe, it is possible accurately to regulate the combustion air ratio even with varying engine output. Furthermore, when varying the heating value of the gas (at least without any measurement of the heating value), it is intended to allow regulation of the lambda value within the required lambda range.